Add workaround for OS X 10.11

OS X 10.11 introduced a new USB framework. Apple broke the automatic selection
of the second configuration so no network device shows up.

This introduces an ev3dev.txt file in the boot partition that contains options.
By adding the line `cdc_only=true` to this file, only the CDC configuration
will be used and we will have a working USB network device on OS X.

Issue ev3dev/ev3dev#471

scripts/ev3-usb.sh

@@ -50,6 +50,14 @@ ev3_usb_up() {
     ms_compat_id="RNDIS" # matches Windows RNDIS Drivers
     ms_subcompat_id="5162001" # matches Windows RNDIS 6.0 Driver
 
+
+    tmpdir=$(mktemp -d)
+    mount /dev/mmcblk0p1 $tmpdir
+    if [ -f $tmpdir/ev3dev.txt ]; then
+        cdc_only=$(grep -i "^cdc_only=\(true\|yes\|1\)" $tmpdir/ev3dev.txt | cut -b 10-)
+    fi
+    umount $tmpdir
+
     if [ -d ${g} ]; then
         if [ "$(cat ${g}/UDC)" != "" ]; then
             echo "Gadget is already up."
@@ -76,34 +84,46 @@ ev3_usb_up() {
     # Windows to be first. The second will be CDC. Linux and Mac are smart
     # enough to ignore RNDIS and load the CDC configuration.
 
-    mkdir ${g}/configs/c.1
-    echo "${attr}" > ${g}/configs/c.1/bmAttributes
-    echo "${pwr}" > ${g}/configs/c.1/MaxPower
-    mkdir ${g}/configs/c.1/strings/0x409
-    echo "${cfg1}" > ${g}/configs/c.1/strings/0x409/configuration
+    # There is a bug in OS X 10.11 that makes Mac no longer smart enough to
+    # use the second configuration. So we've added the cdc_only check to
+    # work around this.
+
+    if [ -z $cdc_only ]; then
+
+        # config 1 is for RNDIS
+
+        mkdir ${g}/configs/c.1
+        echo "${attr}" > ${g}/configs/c.1/bmAttributes
+        echo "${pwr}" > ${g}/configs/c.1/MaxPower
+        mkdir ${g}/configs/c.1/strings/0x409
+        echo "${cfg1}" > ${g}/configs/c.1/strings/0x409/configuration
+
+        # On Windows 7 and later, the RNDIS 5.1 driver would be used by default,
+        # but it does not work very well. The RNDIS 6.0 driver works better. In
+        # order to get this driver to load automatically, we have to use a
+        # Microsoft-specific extension of USB.
+
+        echo "1" > ${g}/os_desc/use
+        echo "${ms_vendor_code}" > ${g}/os_desc/b_vendor_code
+        echo "${ms_qw_sign}" > ${g}/os_desc/qw_sign
+
+        # Create the RNDIS function, including the Microsoft-specific bits
+
+        mkdir ${g}/functions/rndis.usb0
+        echo "${dev_mac1}" > ${g}/functions/rndis.usb0/dev_addr
+        echo "${host_mac1}" > ${g}/functions/rndis.usb0/host_addr
+        echo "${ms_compat_id}" > ${g}/functions/rndis.usb0/os_desc/interface.rndis/compatible_id
+        echo "${ms_subcompat_id}" > ${g}/functions/rndis.usb0/os_desc/interface.rndis/sub_compatible_id
+    fi
+
+    # config 2 is for CDC
+
     mkdir ${g}/configs/c.2
     echo "${attr}" > ${g}/configs/c.2/bmAttributes
     echo "${pwr}" > ${g}/configs/c.2/MaxPower
     mkdir ${g}/configs/c.2/strings/0x409
     echo "${cfg2}" > ${g}/configs/c.2/strings/0x409/configuration
 
-    # On Windows 7 and later, the RNDIS 5.1 driver would be used by default,
-    # but it does not work very well. The RNDIS 6.0 driver works better. In
-    # order to get this driver to load automatically, we have to use a
-    # Microsoft-specific extension of USB.
-
-    echo "1" > ${g}/os_desc/use
-    echo "${ms_vendor_code}" > ${g}/os_desc/b_vendor_code
-    echo "${ms_qw_sign}" > ${g}/os_desc/qw_sign
-
-    # Create the RNDIS function, including the Microsoft-specific bits
-
-    mkdir ${g}/functions/rndis.usb0
-    echo "${dev_mac1}" > ${g}/functions/rndis.usb0/dev_addr
-    echo "${host_mac1}" > ${g}/functions/rndis.usb0/host_addr
-    echo "${ms_compat_id}" > ${g}/functions/rndis.usb0/os_desc/interface.rndis/compatible_id
-    echo "${ms_subcompat_id}" > ${g}/functions/rndis.usb0/os_desc/interface.rndis/sub_compatible_id
-
     # Create the CDC function
 
     mkdir ${g}/functions/ecm.usb0
@@ -112,9 +132,11 @@ ev3_usb_up() {
 
     # Link everything up and bind the USB device
 
-    ln -s ${g}/functions/rndis.usb0 ${g}/configs/c.1
+    if [ -z $cdc_only ]; then
+        ln -s ${g}/functions/rndis.usb0 ${g}/configs/c.1
+        ln -s ${g}/configs/c.1 ${g}/os_desc
+    fi
     ln -s ${g}/functions/ecm.usb0 ${g}/configs/c.2
-    ln -s ${g}/configs/c.1 ${g}/os_desc
     echo "${device}" > ${g}/UDC
 
     echo "Done."